Apple finally explained iPhone 6s battery issue followed by unexpected shutdowns. A few days ago, the company started iPhone 6s battery replacement program. At the time, they did not give any explanation on what is causing the issue. They gave it now, but only on its Chinese website.

“We found that a small number of iPhone 6s devices made in September and October 2015 contained a battery component that was exposed to controlled ambient air longer than it should have been before being assembled into battery packs. As a result, these batteries degrade faster than a normal battery and cause unexpected shutdowns to occur.”, the company says in a statement on its Chinese website.

They added this is not a safety issue. Apple explains that the fact phones are shutting down are created in order to protect the hardware of the device.

“We also want our customers to know that an iPhone is actually designed to shut down automatically under certain conditions, such as extremely cold temperature. To an iPhone user, some of those shutdowns might seem unexpected, but they are designed to protect the device’s electronics from low voltage.”, the statement reads.

The issue only affects the iPhones made in September and October 2015. If you are among the users experiencing this problem, you can visit Apple’s official website and see if your iPhone is eligible for a free battery replacement program.

It is pretty simple. All you need to do is submit the serial number, which you can find by going to Settings > General > About. Enter the serial number, and you will see if your device’s battery can be replaced for free. Also, if you already replaced the battery of your iPhone 6s, Apple is offering a refund.